The document WO 99/32583 discloses a method and apparatus for gasification of solid carbonaceous material. A circulating fluidized bed (CFB) gasifier is used which is described in FIG. 1 of the document and comprises a pyrolysis reaction chamber (1), a particle separator (2) for separation of char-containing particles from the outlet gas (32) of the pyrolysis reaction chamber, a char reaction chamber (3), having at least one inlet for particles from the particle separator, and means for further recirculating particles from the char reaction chamber to the pyrolysis-reaction chamber. The dual chamber CFB gasifier's operation may be controlled in different ways. The disclosed apparatus and process functions well at comparatively low temperatures, beneath 750° C. Due to this success at unusually low and well controlled temperatures, this system is particularly well suited for organic biomass, waste streams and energy crops which contain a relatively high concentration (>0.2%) of elements such as Potassium and Phosphorus, which tend to exist in or form low melting ash components.
In large scale gasification using this multi-chamber, low temperature system, temperature control is an important operational consideration. The endothermic pyrolysis reaction is driven by heat provided by recirculating particles. This effectively requires that temperature in the char reaction chamber be maintained at least 30 or at least 50° C. higher than that which prevails in the pyrolysis chamber. Ideally, temperature in the char reaction chamber can be maintained at a level as close as practicable to, but still beneath, the ash agglomeration temperature.
Fine tuned temperature regulation generally relies on control of bed particle recirculation. We have discovered that, in this system, not only the char conversion but also control of bed particle circulation is much improved by introducing at least one secondary char reaction chamber which during operations contains a bed of particles that is fluidized by a predominantly vertical upwards flow of gas and that has a level substantially beneath the bed level in the primary char reaction chamber. Where fluidizing gas is added in the secondary char reaction chamber, it is necessarily added beneath the level of fuel addition in the pyrolysis chamber. The gas flow through and out of the secondary char reaction chamber acts as to reduce the bed densities in the secondary char reaction chamber as well as in the pyrolysis chamber thereby increasing the overall particle circulation out of the pyrolysis chamber, and in turn from the primary char reaction chamber, without requirement for substantial addition of fluidizing gas to the pyrolysis chamber itself. In this manner, it is possible to fully or nearly avoid introduction of air into the pyrolysis chamber, and the associated reduction in heating value of the product gas.
Even where air is used as fluidizing gas, when this is added to the secondary char reaction chamber, no unwanted oxygen enters the pyrolysis chamber because it is consumed in the secondary char reaction chamber. Unwanted temperature increases from primarily oxygen reacting exothermic with char can be avoided through increased introduction of steam in the secondary char reaction chamber. This added steam contributes to “productive” cooling, whereby endothermic char conversion is achieved with associated production of combustible, water-derived hydrogen gas. The addition of air and steam in the secondary char reaction chamber improves overall char conversion efficiency and decreases the load of fine recirculating char particles experienced by the particle separator.
Surprisingly, all of these benefits of a serial, secondary char conversion system, most notably including “productive” temperature moderation based on endothermic char conversion, can be achieved in a secondary char reaction chamber that comprises a comparatively small percentage of the total volume of the primary char reactor system. By selectively maintaining a relatively low nominal gas velocity and a relatively high ratio of steam to air in the fluidizing gas input, a comparatively small secondary char reaction chamber can contribute to efficient circulation control in a multi-chamber, low-temperature CFB system.